A UN-backed offensive to destroy a Hutu rebel group in eastern Democratic Republic of Congo has had disastrous humanitarian consequences, with more than 1,000 civilians killed, 7,000 raped, and 900,000 forced from their homes.

A group of 84 local and international organisations today described the human cost of the attempt to defeat the Democratic Forces for the Liberation of Rwanda (FDLR) as "unacceptable and disproportionate to the results it has achieved".

With 1,071 FDLR soldiers having given up their arms since the start of military action in January, it effectively means that one civilian has been killed, seven women and girls raped and 900 people displaced for every rebel disarmed, according to the Congo Advocacy Coalition (CAC).

At least 6,000 houses have been burned down by rebels, some as recently as September.

Although many of the killings were carried out by Rwandan Hutu militiamen in retaliation for the offensive, Congolese government soldiers have also targeted civilians, the report said.

Marcel Stoessel, head of Oxfam in Congo, said military action needed to be immediately suspended. "This appalling violence is no accident. It is the result of the UN-backed Congolese military operation against the FDLR militia," he said. "It is a strategy that is being supported in capitals and in the highest echelons of the UN."

Eastern Congo has been one of the world's worst places for civilians since the 1990s, when perpetrators of the genocide in Rwanda fled across the border, and local guerrillas and foreign soldiers battled for control of lucrative mineral deposits. Though the situation has improved in some areas, aid workers say that the overall humanitarian situation is worse than a year ago, when the advance of Laurent Nkunda's Tutsi rebels towards the strategic town of Goma raised international alarm.

Nkunda had been backed by Rwanda, which supported his fight against the FDLR because the Hutu militia's leadership included participants and planners of the 1994 genocide.

But in January Rwanda arrested Nkunda, judging that he had become a liability, and his forces were integrated into the Congolese army. In a positive sign of co-operation, Rwanda and Congo then launched a brief, joint operation against the FDLR. Since March, UN peacekeepers have provided tactical expertise and more than £3.8m in logistical support to the Congolese army offensive, known as Kimia II.

The CAC said the FDLR had deliberately killed many civilians in retaliation to that military operation, and remained a potent force. Estimated to have had 6,000 to 7,000 fighters at the beginning of the year, the FDLR has since recruited more combatants to replace some of those disarmed and repatriated to Rwanda, it added.

Government soldiers, who are poorly paid and lack discipline, have also targeted local communities through killings and widespread rape during the offensive.

"We're seeing more cases of mutilation, extreme violence, and torture in sexual violence cases against women and girls, and many more of the victims are children," said Immaculée Birhaheka of Promotion et Appui aux Initiatives Féminines, in North Kivu province.

In its statement, the CAC said that while disarming the FDLR should remain a top priority for the Congolese army and UN peacekeepers, civilian protection needed urgent improvement. The 3,000 additional UN peacekeepers authorised by the security council last November are only now arriving in eastern Congo, while extra helicopters and intelligence-related support have not materialised.

Anneke Van Woudenberg, of Human Rights Watch, said the UN needed to use its leverage to ensure abusive officers in the Congolese army are removed from command positions. The CAC also urged action against FDLR leaders based in Europe.